This invention relates to catalysts suitable for aromatization of paraffins and olefins, a method of making such catalysts, and an aromatization process using same.
Aromatic hydrocarbons are important chemicals in the petroleum industry. The most commercially valuable aromatics are the BTX compounds such as benzene, toluene, ethylbenzene and xylenes. Aromatics are currently produced by catalytic cracking of naphthas, catalytic reforming of various petroleum feedstocks etc. They can also be produced by catalytic conversion of alcohols, particularly methanol, or olefins. The catalysts used in these processes, like methanol-to-gasoline or MTG developed by Mobil Oil, or olefins-to-gasoline-and-distillate (MOGD) or M2 Forming, also developed by Mobil Oil, belong to the pentasil zeolite family whose most important member in terms of industrial applications is the ZSM-5 zeolite structure. The latter is a tridimensional crystalline aluminosilicate having strong acid sites and whose intermediate pore (or channel) system displays a reaction shape and size selectivity which leads to the production of substantial amounts of monoaromatics. In the M2 Forming and MOGD processes, the ZSM-5 zeolite is used in its acid form alone without any cocatalyst.
Pentasil zeolites have been known for a number of years. The ZSM-5 zeolite is described and claimed in U.S. Pat. No. 3,702,886, while ZSM-11 is described in U.S. Pat. No. 3,709,979.
Zinc oxide and gallium oxide are known as modifiers of zeolite catalysts for the aromatization of hydrocarbons, particularly light alkanes such as ethane, propane and butane. In particular, the so-called Cyclar process developed jointly by British Petroleum and United Oil Products, is commercially employed for the conversion of propane and butanes to aromatics. The catalyst used is a gallium ZSM-5 zeolite. Such catalysts are usually prepared by wet impregnation of a gallium salt onto the acidic surface of the zeolite, or by ion-exchange.
In U.S. Pat. No. 4,975,402 (Le Van Mao et. al.), it has been observed that ZnO species did not require to be adjacent to the zeolite acid sites to be efficient as aromatization cocatalysts. Indeed, by simply setting the zinc bearing cocatalyst in physical contact with the zeolite particles and embedding the two catalyst components in an inert carrier, hybrid catalysts exhibiting high performance in the aromatization of light olefins and paraffins were obtained.